The mutagenic activity of ethylmethane sulfonate (EMS) as a function of its DNA alkylating ability is being study in Salmonella typhimurium. The mutagenic activity of EMS in the base-pair substitution strain G-46 and its repair deficient derivatives (TA1950, [uvrB]; TA92, [pKM101]; TA2410,[uvrB, pKM101] are being compared. The increases in mutation frequencies in these strains will be related to the levels of DNA ethylation and removal. This will provide a reference for the effects of the various repair deficiencies on EMS-induced mutagenesis and will provide data to allow us to relate treated dose to delivered dose to mutagenic response.